Declare their treasuries worthless and default on them for picking a fight with us.
That could have potentially worked in a world where the world opinion of the US was favorable. Heck it's not even the world opinion, it's the other countries which hold substantial amounts of the US debt. I think that Japan would go along with it, but unfortunetly I don't think that you will see any EU member nations go with that plan, they simply put are not happy with us and regardless of your personal opinion of them, they do matter when it comes to matters of international finance. I think that the country's credit rating would drop into the garbage. The ramifications would be felt for years possibly 10's of years.